The present invention relates generally to industrial sewing machines, and in particular to a needle position detector.
The needle position detector plays an important role of an industrial sewing machine due to the fact that the needle position information is used to control the solenoid-operated clutch and brake arrangement which is essential to a variety of industrial sewing functions including variable speed control and stoppage at desired needle positions. A high degree of precision and reliability is thus required of the needle position detector to meet the requirements of the industrial application. In order to monitor the instantaneous position of the needle, the detector is connected to the armshaft of the sewing machine with which the needle is driven and mounted on the sewing machine head. Being located in a position adjacent to the operator, the detector is required to be compact in design to allow space for sewing operations.
Conventional needle position detectors can be classified into a number of types including an electromagnetic system, oscillator type and an electrooptical system. In the electromagnetic system a permanent magnet is attached to a rotary part of the sewing machine so that its opposite poles correspond respectively to the upper and lower needle positions and a Hall generator is mounted stationarily with respect to the magnet. In another electromagnetic system, a ferromagnetic member is attached to the rotary part and the permanent magnet and Hall generator are mounted stationarily with respect to the rotating ferromagnetic member to generate a signal as the latter traverses the magnetic flux. However, shortcomings inherent in such electromagnetic systems are difficulty in determining the critical value of magnetic flux since a lower critical value will render the detector less immune to external magnetic flux and a higher critical value will require the use of a permanent magnet containing a costly rare earth element. Arranging the permanent magnet so that its opposite poles rotate at 180 degrees apart, while advantageous for keeping the size of the detector to a minimum, is disadvantageous due to the fact that for detecting upper and lower needle positions two of such magnets are required which must be spaced a distance sufficient to allow the Hall generator to sharply distinguish between adjacent poles. In another prior art electromagnetic detector, the magnet and Hall generator are mounted on a stationary support between which a slitted iron rotary disk is arranged to rotate to act as an interceptor. This type of system requires that the magnet and Hall generator be spaced a substantial distance apart for satisfactory operation and that the magnet be composed of a costly rare earth element to generate a strong magnetic field. Thus, the goal of compactness and economy has not yet been accomplished with conventional needle position detectors of the magnetic type.
Needle position detectors of the oscillator type, on the other hand, comprise a flux generating coil and a sensing coil which are mounted in an oppositely facing relation, and a slitted rotary iron disk which is arranged to rotate through the space between the two coils to alter the frequency of oscillation. Needle position detectors of the optoelectrical type currently include a set of a light emitting and sensitive elements which are facing to each other and between which is provided a rotary interrupter. However, in either of these prior art systems it is difficult to achieve compactness.
The problem of compactness is compounded by the fact that industrial sewing machines are operated in a wide range of speeds according to the depression of a foot pedal and this operating speed must be controlled with a high precision in a closed loop by sensing the actual speed of the sewing machine. Being coupled to the armshaft, the speed sensor reduces the space allowed for the needle position detector.